Filter elements such as these are readily available on the market in a plurality of embodiments. These filter elements are used when inserted in the filter housing to clean the supplied dirty fluid, for example in the form of a hydraulic medium, by using the filter material of the filter element, and to return the fluid cleaned in this way from the filter housing to the fluid circuit, in particular the hydraulic circuit. If after a plurality of filter cycles the filter material is clogged with dirt such that it is used up, the filter element is removed from the housing and is replaced by a new one. To increase the filtering surface, modern filter elements have a filter mat web folded in a star shape and surrounding an inner or outer support tube in a concentric circle, against which the filter material with its filter folds folded up in a star shape can be supported. Especially when flow takes place through the filter element from the outside to the inside, can the filter folds be supported on the support tube during filtration. The filtration direction can be reversed, that is, from the inside to the outside. Furthermore, backflushing processes with already cleaned fluid are also conceivable to clean the filter material clogged by dirt. The filter material itself has a multilayer mat structure, the layer structure being dependent on the filtration task to be performed. Generally, layer structures are used with clean filtering and draining layers, composed of native fibers, plastic fibers, such as meltblown fibers, glass fibers, and the like. To ensure a fixed association of layers against one another, the two cover layers of the filter material can be a fine-mesh wire fabric seeking to prevent washout of the fiber material.
The disadvantage of filter materials folded in a star shape is that the individual filter folds can be deformed during filtration and come to rest on one another, forming a block. This deformation reduces the surface effective for filtration and consequently degrades filter performance. These faults occur especially in fluids with high viscosity, as generally occur for example in a cold start of a fluid system or hydraulic system. Blockage of the folds causes a degradation of drainage and ultimately leads to increased pressure losses at a reduced dirt holding capacity and a reduced service life for the filter element.
To help avoid unwanted deformation of the folds and formation of fold blocks, the prior art (WO 01/85301 A1) has already suggested surrounding the concentrically arranged filter material with its filter folds on the outer peripheral side with a support. The support is a type of hollow cylinder with its outer periphery on the inner periphery of the outer housing part of the filter element. On the inner peripheral side, the support forms individual projections engaging the spaces between adjacent filter folds to prevent them from coming to rest on one another in an undesirable manner or even from being folded over. Since in the known solution the fluid-permeable support is formed from a type of porous sponge (available under the trademark “SIF” from Foamex International Inc., 10000 Columbia Avenue, Linwood, Pa. 19061), it has high inherent elasticity. With a certain pretensioning, the adjacent filter folds are then kept at a distance from each other and can engage the depressions of the otherwise cylindrical foam support.
With the known solution, very good supporting effects can be achieved and failure of the filter element, even at higher fluid viscosities, as occur in cold starts, is precluded in this way. In addition to increasing the operating reliability, the known solution can also be economically implemented.
This known solution has been found to be still susceptible to blockage. This blockage is caused by ageing products in the fluid which can lead to the entire filter element becoming choked and unusable. Thus, among other things, the types of oil used in wind power plants are characterized by a high content of additives to be able to form very good tribologic properties. The disadvantage is that in the aged state of the oil the additives occur as ageing products or oil residues which are not oil-soluble and which then cement the surface of the filter material. In this respect, the filter element can then only make available a part of the actual dirt holding capacity in filtration. This situation in turn leads to increased pressure losses and to reduced service life of the optimized filter element with support means.